Recombinant DNA technology has permitted the expression of exogenous or foreign (heterologous) proteins in bacteria and other host cells. Under some conditions, and for some proteins, these heterologous proteins are precipitated within the cell as "refractile" bodies. The present application concerns procedures for recovering these heterologous proteins and for restoring them, if necessary, to their active forms.
A large number of human, mammalian, and other proteins. including, for example, human growth hormone, (hGH) bovine growth hormone (bGH) and a number of interferons have been produced in host cells by transfecting such cells with DNA encoding these proteins and growing resulting cells under conditions favorable to the expression of the new heterologous protein. Viral coat proteins, such as capsid proteins of foot and mouth disease (FMD) virus and the surface antigenic protein of hepatitis B virus (HBsAg) are still other examples of heterologous proteins which have also been produced in suitable recombinant DNA engineered hosts. The heterologous protein is frequently precipitated inside the cell, and constitutes a significant portion of the total cell protein.
In a large number of important cases, such as those of hGH, porcine growth hormone (pGH), bGH, FMD, and fibroblast interferon (FIF), it has been observed that the heterologous proteins produced are not only present in large quantity, but are precipitated within the cell in the form of "refractile" bodies. The term "refractile" is used because these bodies can actually be seen using a phase contrast microscope. Under magnifications as low as 1000 fold, these precipitated protein bodies appear as bright spots visible within the enclosure of the cell.
Recovery of the desired protein which is in the form of such refractile bodies has presented a number of problems. First there is the obvious need to separate the refractile protein, which is encased within the cell, from the cellular material and proteins harboring it. Second, it appears that while the refractile body may often consist of a large percentage of the desired heterologous protein, and only a small portion of undesired ones, in some instances there are sufficient protein contaminants that these must be removed to isolate desired polypeptide sequence. Third, and perhaps most troublesome, the refractile body protein is often in a form which, while identifiable as the desired protein, is not biologically active. It is believed that this inactivity is due to incorrect folding or conformation of the heterologous protein either before or after intracellular precipitation, or during the isolation process.
It has now been found that these problems can be overcome by utilizing procedures which, in their various aspects, succeed in removing the contaminating host cellular protein, solubilizing the precipitated refractile protein, and restoring the heterologous protein to a form which is active in biological assays.